Daniel Radcliffe is a very mediocre actor. He has improved a lot, yes, but still I feel he is very wooden and expressionless. He does look like Harry, but he can't portray the vulnerable/emotional side of his character.
Emma is a decent actress, but she overdoes her part at times with melodramatic tones and all that. And she can't portray the vulnerable side of Hermione either; she was terrible in Half Blood Prince, in my opinion.
Rupert is, by far, the best of the trio. He might be the least attractive Potter actor, but he has defined and perfected the role of Ron for ages.
Tom Felton - don't see anything great in his acting and don't really understand the hype around him.
Evana Lynch was perfect as Luna, but I wouldn't call her a great actress. She skimmed into the character of Luna effortlessly because, IMHO, that's her real life personality.
Oh how can I miss Michael Gambon? People say he is wooden, can't act, can't portray the complexity of Dumbledore, etc etc, but IMO, Richard Harris wasn't that great either. I really can't see Richard doing Half Blood Prince or fighting with Voldemort in The Department of Mysteries. Someone like Ian Mckellen should have been casted.
HBC is excellent as Bellatrix! So was Imelda who played Umbridge!